The invention relates to a coated valve for internal combustion engines having a layer which prevents deposits.
The coking of valves is a problem which has long been known, in particular in the case of inlet valves of Otto engines. Coking is understood to mean black, hard deposits which are produced mainly by combustion and decomposition residues of the fuel.
The requirements, which have been increasing for some years, imposed on the emission behavior, higher power outputs and lower fuel consumption, result in engines which are operated with a leaner fuel/air mixture. These requirements, as well as the use of unleaded fuel, result in the valves being more easily coked, and a coking of the valves manifests itself in a more troublesome manner than before.
The negative effects of inlet valve coking include poor cold running, poor gas admission, particularly in the warming-up phase, an increase in the fuel consumption and pollutant emission associated therewith, as well as a drop in power due to interference in the inlet air passage and out-of-true engine running. In addition, coke particles may damage the valve seating surfaces, which results in leaky valves.
Attempts to prevent coking of the inlet valves have included adding additives to the fuel, and also providing the valves with a layer which prevents deposits. Thus, it is shown in German Unpublished Patent Application No. 3,517,914 to provide inlet valves with a polytetrafluoroethylene layer. However, in practice such layers have not resulted in success.
An object of the present invention is to provide a coated valve for internal combustion engines having a layer which prevents deposits, and in which the formation of deposits is reliably prevented even in the long term.
These objects and other objects are achieved according to the present invention, by providing a value with a deposit preventing layer of cerium (IV) oxide. It was discovered that coking substantially ceases with the coating of the valve with cerium (IV) oxide. A method of coating a valve with the deposit preventing layer is also provided.
It is contemplated that the layer be applied to the valve by any desired manner and means. In certain preferred embodiments, it is preferable to produce the layer by plasma jet spraying of cerium (IV) oxide. In certain preferred embodiments, the cerium (IV) oxide powder, which expediently has a particle size of 40-100 .mu.m may contain in addition small quantities of other oxides, such as TiO.sub.2, Cr.sub.2 O.sub.3, V.sub.2 O.sub.5, FeO etc. Since, however cerium (IV) oxide represents the active constituent of the layer, the layer should contain as much cerium (IV) oxide as possible, and at least 90% by weight.
According to advantageous features of certain preferred embodiments of the invention, the thickness of the cerium (IV) oxide layer applied should be about 0.1 to 1.5 mm. In certain particularly preferred embodiments, the layer thickness is between 0.15 and 0.4 mm.
It is contemplated that the adjustment of the jet spraying parameters in plasma jet spraying is critical for the mechanical properties of the layer applied. The quality of the layer can be influenced by varying the jet spraying distances. A small jet spraying distance produces mainly a cerium (IV) oxide layer of a porous and soft nature. Larger jet spraying distance produces higher proportions of Ce.sub.2 O.sub.3, which does not have a catalytic action but is of a harder and firmer or more dense nature. In certain preferred embodiments, these proportions of Ce.sub.2 O.sub.3 can be converted into cerium (IV) oxide by a simple oxidation treatment, for example heating for 10 to 20 minutes at 400.degree. C. in air, without the mechanical properties of the layer being affected.
According to advantageous features of certain preferred embodiments of the invention, to improve the adhesion of the cerium (IV) oxide layer, an adhesion layer is provided in a manner known per se between the cerium (IV) layer and the valve material. Such adhesion layers are often used in flame jet spraying or plasma jet spraying technology, and include in general, sprayed-on layers of a nickel/-or optionally cobalt-containing nickel/chromium/aluminum alloy. These adhesion promotion layers are usually applied in a layer thickness of between 0.05 and 0.1 mm.
According to other advantageous features of certain preferred embodiments of the invention, the cerium (IV) oxide layer covers the entire valve with the exception of the seating and guide surface. However, in certain preferred embodiments, the valve has the cerium (IV) oxide layer only on the surface regions which are endangered by deposits. These are, in particular, the rear part of the valve head, and also the part of the valve stem adjacent thereto.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.